Lead is an important commodity in the United States wherein approximately 5.times.10.sup.5 metric tons of lead per year are consumed. Lead is used in a number of industrial applications including storage batteries, pigments, ammunition, solders, plumbing, cable covering, bearings and caulking, to name a few. Lead's physical properties, which include a low melting point, a high density, low strength, and chemical stability in air, water, and earth, make lead an element of choice for many industrial applications. Because lead is so widely used, lead poisoning is a worldwide problem.
People are exposed to lead from a variety of sources. These include high lead content in soil due to naturally occurring lead concentration, exposure to industrial waste, or fallout from leaded gasoline exhaust. The high lead content in soil is particularly a health problem when compounded with the inability to wash lead from vegetables or fruits. Studies have further found high levels of lead in foods preserved using lead-soldered cans and bottles. Occupational exposure represents a significant risk for workers. Industries, such as automobile manufacturing, ceramic manufacturing, alcoholic beverage manufacturing, and paint manufacturing, all have documented cases of workers suffering from lead poisoning. Lead dust carried on the person of a worker from an industrial site to their home incurs additional risk to children and other family members. Furthermore, lead is being detected in drinking water in numerous cities. This is due to leaching from soldering the pipes of the water distribution system. For each of the above examples, there is a time delay before the symptoms of lead poisoning are recognized because the poisoning is cumulative.
Lead is poisonous because its presence in a person's body inhibits many enzymatic reactions, such as an adverse affect on heme synthesis. Lead intoxication can cause chromosomal aberrations and mutagenicity although there is no evidence that lead is carcinogenic. The OSHA lead standard requires any worker whose blood lead level exceeds 50 micrograms per deciliter (.mu.g/dL) to be removed from exposure. However, 30 .mu.g/dL to 40 .mu.g/dL is also believed to pose a significant risk. The Environmental Protection Agency suggests lead in drinking water to be less than 1.5 .mu.g/dL. Thus, the analytical challenge is to easily and quickly determine the source of chronic lead exposure.
Occupational or natural exposure to lead induces a risk that may not be altogether avoidable. However, during the 1980's, approximately three million children between the ages of six months and five years were found to have poisonous lead levels in their blood. The signs of lead toxicity include headaches, dizziness, abdominal pain, coma, convulsions, manic behavior, and delirium. Virtually all of these cases of lead poisoning could have been prevented through education, awareness, and adequate testing of the environment. A vital link in preventing cases of childhood lead poisoning is determining the degree of lead in the paint used in the child's home.
Analysts measure lead through spectroscopic, photometric and electrochemical techniques. By far the most widely employed method is graphite furnace, Atomic Absorption Spectroscopy (AAS). A sample is digested in acid and placed in a graphite furnace. The characteristic spectral absorption line is monitored and using Beer's Law, related to lead concentration. AAS instruments are expensive and too large for field use. Another measuring technique, X-Ray Fluorescence (XRF) analysis, consists of subjecting a sample to x-rays. The x-rays excite and expel electrons from a given atomic level. By measuring the resulting fluorescence line, the lead concentration can be determined. XRF analysis is extremely sensitive so that this technique suffers from matrix effects. The operator must closely match the sample and reference matrix to achieve accurate results. Even with a highly trained operator, obtaining accurate results is difficult when sampling unknown paint residues.
Researchers have developed some strategies to minimize the matrix problem for XRF analysis as discussed in an article by Kuntz et al. in the Journal of Coating Technology, 1982, 54(687), p. 63. For example, a few laboratories use the photometric method. The photometric method, as discussed in an article by Henderson et al. in the journal Analytical Chemistry, 1961, 33, p. 1173, consists of reacting uncomplexed lead with dithizone, extracting into chloroform, and measuring the absorption maxima at 520 nanometers (nm). This method is deficient because of interferences with other metal ions. Therefore, some experimenters add potassium cyanide (KCN) or other agents (EDTA) to mask the other metals, as discussed in the book by Jungreis entitled Spot Test Analysis, John Wiley and Sons, N.Y. 1985, p. 205. The requirement for sample work-up and the interferences make photometric analysis unsuitable for field studies.
A seldom used electrochemical technique is the lead Ion Selective Electrode (ISE) method as discussed by Vesely et al. in a book entitled Analysis with Ion-Selective Electrodes, John Wiley and Sons, 1991, p. 325. The lead ISE is like a pH electrode, except a surface specific for lead is used. Poor reproducibility and selectivity over other solution species, like protons and dissolved oxygen, interfere with the results from the lead ISE method.
Another technique used by some laboratories is the Anodic Stripping Voltammetry (ASV) technique for elemental analysis, such as lead in blood and other bodily fluids, see the articles by Morrell et al. in Clinical Chemistry, 1976, 22, p. 221 and Stauber et al. in Science of the Total Environment, 1988, 74, p. 235. A monograph that reviews ASV with respect to applications for determining toxic metal ions, inorganic compounds, and both ionic and neutral organic compounds has been written by Wang, in Stripping Analysis, VCH Publishers, 1985. He also details the use of the analogous Cathodic Stripping Voltammetry that is used for a range of similar electro-active substances. The ASV technique typically employs a mercury electrode with a potential impressed thereon causing diluted lead within a solution to be first reduced to metallic lead and then dissolved into the mercury electrode. The partition of lead from solution to the mercury electrode effectively concentrates the lead. Next, an operator using a potentiostat "scans" the potential of the mercury electrode, i.e., constantly adjusts the potential to a series of different, spaced positive potentials, causing oxidation of the dissolved lead back to free ions and its stripping from the mercury electrode. Each metallic element strips from the mercury electrode at a unique potential. Thus, the identity of the metal is determined from the potential at which the element strips out of the mercury. However, the scanning step alone results in a background current, caused by the potential applied to the mercury electrode and the electrical resistance of the analyte solution. The scanning step is also responsible for the faradaic current that corresponds to the presence of lead or other trace elements being oxidized and stripped off from the mercury electrode. Since a simple potential scan can result in a very large background current which ultimately swamps the faradaic signal, researchers often make use of a differential pulse waveform. Here, a voltage pulse is applied to the simple scan. A sophisticated electronic circuit samples the current at discrete times along the pulse, and performs a differential operation of the current. The differential can reduce much of the background current while more clearly resolving the faradaic current. The differential pulse scanning step requires sophisticated electronic circuitry that is both expensive and is packaged in a relatively large case. This technique is commonly called Differential Pulse Anodic Stripping Voltammetry.
The electrochemical steps of the ASV technique generate an analytical signal that enable an operator to test specifically for the presence of lead by selecting the potentials of reduction and oxidation, and a method to preconcentrate dilute lead solutions. Although researchers have reported using the ASV technique for lead in paint, see Lai et al. in Analyst, 1978, 103, p. 1244, the ASV technique for lead analysis has not gained widespread acceptance. As documented by Morrell et al. in Clinical Chemistry, 1976, 22, p. 221, the ASV technique can be as accurate, precise, and as fast as the AAS method for lead analysis. Still, most laboratories use the AAS method, partly due to the difficulties in using the ASV technique for multi-element analysis when both method development and a knowledgeable operator are needed.
A relatively new research area in electroanalytical chemistry is the use and exploitation of microelectrodes to replace traditional, i.e., analytical, electrodes with dimensions in the range from 1.0 cm.sup.2 to 0.02 cm.sup.2. Neurobiologists began to use electroanalytical techniques to investigate brain chemistry and developed the need for ever-smaller electrodes for in-situ work. By the early nineteen eighties, experimenters in other scientific fields discovered novel properties of electrodes whose diameter ranged from 100's of microns to submicron dimensions, see the article by Fleischmann et al., editors of Ultramicroelectrodes, Datatech Systems, Inc. 1987. For example, the low surface area charging of the small electrodes improved the signal to noise ratio during measurements. The need for a small charging current also meant faster kinetic measurements were accessible and measurements could be performed in solutions without supporting electrolytes.
There are various physical forms for microelectrodes. Researchers found that microelectrode properties could not only be achieved with disks of carbon or metal fibers embedded in glass pipettes, but cylinders, drops, rings, bands, lines, and arrays of lines could all be constructed with small enough dimensions to maintain microelectrode behavior. The configuration referred to as arrays of lines is a series of lines formed using, for example, photolithography or silk-screened patterns. One disadvantage of the listed microelectrode configurations, except for the arrays of lines, is that the currents developed during the measurement are on the order of nanoamps, which require special instrumentation for detection thereof.
Sanderson et al. describe the benefits of using an array of twin interdigitated electrodes in an article appearing in Analytical Chemistry, 1985, 57, p. 2388. By using one set of electrodes as a "generator" and the other set of electrodes as a "detector", they show enhanced signal-to-noise ratios (compared to a macro electrode) and, because the array is composed of long bands of electrodes of very small width, they show current levels detectable by conventional electrochemical instrumentation. Thorman et al. use an interdigitated array as a detector for isotachophoresus, see the article in Separation Science and Technology, 1984-1985, 19, p. 995. This article discusses the researcher's control of the potential of each individual electrode. Others have started to investigate the possibility of using interdigitated arrays for flow-through analysis, see Bixtler et al. in Anal. Chim. Acta, 1986, 187, p. 67.
Cushman et al., as discussed in Anal. Chim. Acta. 1981, 130, p. 323, were the first to show the feasibility of the ASV technique performed at a microelectrode. Others have continued to develop the methodology of performing the ASV technique at carbon fiber microelectrodes, see Shulze et al. in Anal. Chim. Acta. 1984, 159, p. 95, and Wehmeyer et al. in Anal. Chem. 1985, 57, p. 1989. The advantages of incorporating the ASV technique with microelectrodes are similar to those discussed before and include enhanced signal to noise and shorter analysis times, as disclosed by Sottery et al. in Anal. Chem. 1987, 59, p. 140; and no requirement for a bulk solution of analyte in using the ASV technique, see Heineman et al. Anal. Chem. 1977, 49, p. 1792, wherein a thin-layer electrochemical cell was employed with a conventional electrode, the ASV technique was used to measure lead, cadmium, and zinc in a 60 .mu.L sample volume. Others have developed flowing systems for use in clinical labs, see the article by Wang in Am. Lab. 1983, 7, p. 14 and in the field, see the article by Zirion in Environ. Sci. Technol. 1978, 12, p. 73. Some of these systems offer computer aided automation and control of the process for the ASV technique. All of these systems are large, relatively expensive, and difficult to operate.